Hilux 4wd issues

Hi everyone, I have an '08 Hilux SR5 with ADD. Everytime i try shifting from h2 to h4 i am met with a lot of resistance. The van is an automatic and I dont understand what is causing the problem.

The 4wd lever just refuses to move. Sometimes if i just start the van and put it in neutral or even drive the 4wd works as it should and the 4wd light comes on the dashboard and the lever moves freely.
However after driving for some time it does not engage. Even if the shifter itself moves from H2 to H4 the light does not come on the dashboard.

What may be some possible reasons as to why this is happening? The manual clearly states that once travelling under 80kmh shifting through the high range is no problem.

Update: A bit of research may point to the front diff actuator ceasing up but does not account for the lever resistance.

Update 2: THere was absolutely no problem with the system, it was a lack of knowledge and understanding... The manual did not help much.